Heretic said that Starbases would take 7 months to finish with no downtime between projects.
So half of that is 3.5 months. It will take "less than half", so lets say 3 months.
3 months of all but ignoring your regular starbase projects, with no downtime between the Embassy projects...

Should be substantially faster than that, if things are proportional to starbases. There's only 64,000 XP in the subtiers, compared to 750,000 XP in the starbase ones.

Assuming they stay proportional at all three tiers, time wise, it should be slightly less than tier 2 in all three starbase tracks (75,000 XP), but probably more expensive because you spend half of that at tier 3 costs, not tier 2. Cost wise, this is hitting the point where a starbase is no longer a viable solo project, not the point where they become an unmanageable group project.